1. Field of the Invention
This invention relates to a sheet detecting device and an image forming apparatus provided with the same, and particularly to the construction of sheet detecting means.
2. Related Background Art
There has heretofore been an image forming apparatus of the electrophotographic type or the electrostatic recording type in which for example, a visible image is formed on an image bearing member and this visible image is transferred to a transfer material which is a sheet to thereby obtain an image. And as an example of such an apparatus, there is a polychromatic or monochromatic electrophotographic copying apparatus, a laser beam printer or the like.
Now, in such an image forming apparatus, it is necessary to reliably effect the feeding and conveyance of a transfer material to the image bearing member and the discharge of the transfer material having a visible image transferred thereto and fixed thereon. So, in order to confirm that the feeding, conveyance and discharge of this transfer material are effected reliably, transfer material detecting devices which are a plurality of sheet detecting means are provided on the route of feeding, conveyance and discharge. Further, a transfer material detecting device for detecting the twining or the like of the transfer material onto the roller of a fixing device is also provided immediately behind (near the downstream side) the fixing device for fixing the toner image on the transfer material.
The transfer material detecting devices each comprise a flag which is a first member urged and pivotally moved by the transfer material when the transfer material passes it, and a photosensor which is a detection signal producing portion adapted to be turned on/off by the pivotal movement of the flag and produce a sheet detection signal. This flag is provided with biasing means such as a spring, and after the transfer material has passed, the flag is adapted to be pivotally moved and returned to its position before its pivotal movement by this biasing means.
On the other hand, in the transfer material detecting device of such a construction, when a photosensor cannot be disposed near the flag is construction, this is, when the photosensor cannot be directly turned on/off by the flag, a second member moved with the pivotal movement of the flag is provided and the photosensor is adapted to be turned on/off by the movement of this member.
After the transfer material has passed, the flag pivotally moved and the second member moved with the pivotal movement of the flag are adapted to be returned to their original positions or their original states by the biasing means for biasing the flag.
However, when in the prior-art image forming apparatus provided with the thus constructed transfer material detecting device, the second member moved with the pivotal movement of the flag is provided, the following problem has arisen in some cases because only the flag, or in some cases, only the second member is provided with the biasing means.
The flag and the second member together are biased by a single biasing means and therefore, if the biasing force of the biasing means is made great so that, for example, the flag and the second member can be reliably returned to their original positions, the contact pressure between the flag and the transfer material may become great and the transfer material may be injured or the contacting portion of the flag may be greatly worn. If conversely, the biasing force of the biasing means is made small so as to make the contact pressure between the flag and the transfer material small, the flag and the second member will not be returned to their original positions even after the transfer material has passed.